The subject invention is directed, generally, to optical gain fibers and, more particularly, to optical gain fibers that are pumpable at 980 nm and at 1480 nm, that have low ripple, that have low passive loss, and that are fusion sliceable.
In recent times, the use of optical fiber communications has increased dramatically, and the promise of increased signal transmission speed and clarity makes it likely that the use of optical fibers for signal transmission will continue to grow in the future. Optical fiber technology can be used to transmit a variety of signals. For example, telecommunication, sensor, medical, and video transmissions can all take advantage of optical technology, particularly where virtually unlimited bandwidth and low attenuation are beneficial. Cable television systems are one example where optical fiber technology is providing efficient and economical alternatives to prior coaxial cable distribution schemes.
As optical signals are propagated through an optical fiber the signals becomes attenuated. The degree of attenuation is generally proportional to the length of the optical fiber carrying the signal. Thus, one of the obstacles to long haul transmission systems is the need for signal regeneration. Conventionally, this has been accomplished electrically by transforming the optical signal to electric signals, amplifying the electrical signals using conventional electrical signal amplification techniques, and converting the amplified electrical signals to optical signals. This process introduces several signal manipulations, any one of which can result in the loss of signal quality. As this process is repeated over the course of a long-haul transmission, these losses in the signal quality can give rise to significant problems.
In response to this problem, the use of optical gain fibers has been described. Fiber gain modules use electromagnetic energy to pump an optical signal without first converting the optical signal to an electrical signal. The medium in which such pumping is carried out contains a fiber that is doped with certain rare earth metals, particularly erbium. However, such optical gain fibers have narrow operating windows immediately around 1550 nm, and the flatness of gain over the operating window is fairly low. As a result, while these optical gain fibers are suitable for amplifying optical signals that have a narrow bandwidth centered at 1550 nm, they produce uneven gain in cases where the signals have broader bandwidths or are not precisely centered at 1550 nm.
Accordingly, a need exists for optical gain fibers having a wider gain window and improved flatness across the gain window. The present invention is directed to meeting this need.
The present invention relates to an optical gain fiber which includes a core and a cladding surrounding the core. The core includes erbium, is substantially free of fluorine, and the optical gain fiber is pumpable at 980 nm and at 1480 nm. In addition, the optical gain fiber has ripple of less than about 25% over about a 40 nm wide window or ripple of less than about 15% over about a 32 nm wide window, or both.
The present invention also relates to another optical gain fiber. This optical gain fiber includes a core and a cladding surrounding the core. The core includes erbium, is substantially free of fluorine, and the optical gain fiber is fusion sliceable and has ripple of less than about 25% over about a 40 nm wide window or ripple of less than about 15% over about a 32 nm wide window, or both.
The present invention further relates to yet another optical gain fiber. This optical gain fiber includes a core and a cladding surrounding the core. The core includes oxides of erbium, and is substantially free of fluorine, and the cladding includes oxides of silicon. The optical gain fiber has a passive loss of less than about 0.5% of the peak absorption of the erbium absorption band in the vicinity of 1530 nm. In addition, the optical gain fiber has ripple of less than about 25% over about a 40 nm wide window or ripple of less than about 15% over about a 32 nm wide window, or both.
The optical gain fibers of the present invention have a wider gain window and/or improved flatness across the gain window as compared to conventional optical gain fibers. Furthermore, the optical gain fibers of the present invention has enhanced net gain per unit length at equivalent inversion relative to conventional erbium-doped optical gain fibers. Accordingly, the optical gain fibers of the present invention are useful in amplifying optical signals, particularly signals which need to be repeatedly amplified over the course of a long-haul transmission, without losses in the signal quality that are commonly encountered in conventional optical signal amplifying methods.